1. Field of the Invention
The invention relates to a method of optimizing calls set up in a private telecommunication network including two subnetworks using the QSIG protocol and the Session Initialization Protocol (SIP), respectively.
2. Description of the Prior Art
Conventional private networks including switches for switching circuits or packets of information transported synchronously are known in the art. The standardized QSIG signaling protocol enables the switches to exchange signaling messages, even if they are sourced from different manufacturers.
More recent private networks including routers that route packets of information (voice or data) using the Internet protocol (IP) are also known in the art. The standardized Session Initialization Protocol (SIP) enables the routers to exchange signaling messages.
Private networks including two subnetworks, one switching circuits or packets of information transported synchronously and the other routing packets of information (voice or data) using the Internet Protocol (IP) are also known in the art. The two subnetworks use the QSIG protocol and the SIP, respectively, and are connected by a gateway that handles changes of format for voice and signaling information exchanged between the two subnetworks.
FIG. 1 shows one example of the above kind of private network, which includes:                a subnetwork QSIGN including:                    two interconnected switches P1 and P2 for switching packets of information (voice signals) transported synchronously and using the QSIG protocol to exchange signaling messages (conventional telephone terminals, not shown, are connected to these nodes);            an interactive voice server VS connected to the switch P1 for welcoming calling users and offering in particular to transfer their call to a terminal of the private network; and                        a subnetwork SIPN including:                    an IP router (IPR) connected to the gateway GW and routing packets of information (voice or data) using the Internet Protocol (IP), the gateway handling changes of format for information exchanged between the two subnetworks QSIGN and ISPN; and            Internet terminals IPT1 and IPT2, for example telephones, connected to the router IPR.                        
The switch P2 includes means SW providing the conventional switching functions and means GW constituting a QSIG/SIP gateway.
Assume that a user of the terminal IPT1 calls the terminal IPT2, which is also part of the subnetwork SIPN. The router IPR establishes a session that corresponds to a voice and signaling path connecting the terminal IPT1 directly to the terminal IPT2. There is no particular problem and the path used is the shortest possible.
Assume now that a user of the terminal IPT1 calls the voice server VS to contact an administrative department whose number he does not know, for example. The router IPR sends an SIP command to establish a session corresponding to a voice and signaling path S1-S2 from the terminal IPT1 to the switch P1 with which the voice server VS is associated. However, the SIP is operative only as far as the gateway GW integrated into the switch P2. The gateway GW translates the command from the SIP to the QSIG protocol. The SIP command establishes a first portion S1 of the voice and signaling path, this portion S1 extending from the terminal IPT1 to the gateway GW. The command translated into the QSIG protocol establishes another portion S2 of the voice and signaling path, this portion S2 extending from the gateway GW to the voice server VS. In this way a first call is set up between the terminal IPT1 and the voice server VS via the path S1-S2.
The voice server VS responds to the user of the terminal IPT1, on the same path S1-S2, and proposes a list of administrative or sales departments. The user chooses a department from the list by pressing one or more keys of the terminal IPT1. The voice server VS receives audio signals corresponding to the keys pressed and deduces, for example, that the terminal IPT1 must be connected to the terminal IPT2. It then commands the switch P1 to call the terminal IPT2 and set up a second call. Using the QSIG signaling protocol, the switch P1 commands the setting up of a voice and signaling path S3-S4 from the switch P1 to the terminal IPT2. The QSIG protocol is operative only as far as the gateway GW. The QSIG command sets up a first portion S3 of the path, this portion S3 extending from the switch P1 to the gateway GW. The gateway GW translates the command from the QSIG protocol to the SIP. The translated command commands the router IPR to create a session corresponding to a second portion S4 of the voice and signaling path, this portion S4 extending from the gateway GW to the terminal IPT2.
Finally, the voice server VS commands the switch P1 to connect the first and second calls by connecting the paths S2 and S3. The terminal IPT1 then communicates with the terminal IPT2 via the router IPR, the switch P2, the switch P1, the switch P2 again, and the router IPR.
It is therefore apparent that, if a terminal of the subnetwork SIPN has the benefit of a service offered by a server associated with a switch of the subnetwork QSIGN, the call to that server can be transferred (in some cases) to another terminal of the subnetwork SIPN on a path S1-S2-S3-S4 that is not the optimum because it includes a long detour compared to the shortest path, since the path S1-S2-S3-S4 leaves and then re-enters the subnetwork SIPN.
The object of the invention is to remedy this drawback.